Oil and gas wells are expensive to construct and it is advantageous to operate these wells as efficiently as possible. One way of providing for an increased efficiency in the operation of wells is that equipment located downhole in the well bore is placed under the control of other equipment located on the surface.
The downhole equipment can be measurement sensors which supply useful information for the subsequent operation of the well, which for example might include: data regarding pressure, the nature of the solids and fluids encountered, the temperature, etc. The equipment can also be other controllable or monitoring equipment which is able to supply important instructions from the surface to control various parameters of the well or the reservoir using downhole equipment or devices such as valves, packers, etc.
Electromagnetic (EM) wireless telemetry is a field that is extremely valuable and widely studied in the oilfield. The purpose is to provide wireless bi-directional telemetry, for transferring information from the surface to tools downhole in the well and vice-versa, i.e. to the surface.
Whereas downhole tools might have wireless components in performing their various sensing functions downhole, they often require wired connections to get the sensed information communicated from deep below up to the earth's surface for analysis. Alternatively, the sensed information is stored in memory and only retrieved once the piping structure and associated tools are pulled out of the well. Supplying power at these depth for any wireless communication device does boost the range of such a wireless communication device or antenna, but it requires a remote battery source from which a great deal of power is required downhole. Thus, such wireless communication devices often have a limited range of communications and therefore wired solutions have often proved more practical, especially at great depth. During drilling or drill stem testing operations, the use of a wire is often unpractical and in some cases banned by operators.
Therefore, it is desirable to have a bi-directional telemetry system having a greater EM communication range, which reduces the need for wired communications.
Furthermore, a plurality of different technologies have been tried to produce an optimized bi-directional wireless telemetry system, including for example acoustic through tubing techniques, pressure wave in fluid, etc. However, these other techniques do not appear to offer the range of communication and simplicity that EM brings. Moreover, a distinction needs to be made between so-called ‘open-hole’ and ‘cased-hole’ hole environments, especially where an EM telemetry system is considered.
A conventional open-hole method for measuring a signal downhole is to inject current onto the drill pipe or tubing. Although this method is well-suited to the open-hole system, it is not considered appropriate for a cased-hole situation.
Specifically, a cased-hole system has a casing that lines the walls of the borehole for, amongst other things, maintaining the structural integrity of the drilled borehole. However, the casing is often made from a metal, for example steel, which is highly electrically conductive and thus increases the likelihood of axial leakage of current injected on the drill pipe, resulting in a shorter electromagnetic (EM) dipole length for the antenna. Such leakage can occur either where there is a conductive liquid or substance, for example water, which fills the annular space (or gap) between the drill pipe and casing or in the case of a deviated well, in which the drill pipe and casing come into contact.
In the case of a deviated well, when the drill pipe or casing is sufficiently curved or deviated accumulatively, it is possible that that drill pipe makes contact with the casing, and then the leakage is exacerbated. That is, the injected current will have an extremely short current path with most current simply returning to the current source along the contact area between drill pipe and casing. This ‘short-circuit’ effectively limits any meaningful range for EM communications in a cased hole system.
Therefore it is desirable to overcome the aforementioned disadvantages associated with open-hole injected current antennas when applied to a cased hole environment.